1. Field of the Invention
The present invention relates generally to the field of computer systems, specifically, to a method and apparatus for connecting host controllers to a high-speed bus.
2. Background
Low-speed serial buses have been popular in computer systems because of their low cost. These serial busses couple serial type devices which typically do not have significant bandwidth requirements. An example of a low-speed serial bus is the universal serial "USB" bus. The USB bus supports "USB devices" such as keyboards, joy sticks, pointing devices, mice and audio speakers. Typically, these USB devices do not have significant bandwidth requirements. A USB host controller connects the USB devices to a bus interface unit. The bus interface unit is typically connected to a high speed interconnect bus. In order to minimize the number of USB host controllers needed, a hub or a plurality of hubs may be used to couple several USB devices to each USB host controller. Each hub provides additional ports to the USB host controller.
Each USB host controller has a bandwidth or maximum transfer rate of approximately 12 megabits per second (Mb/s). The maximum transfer rate is set by "Universal Serial Bus Specification 1.0", Jan. 15, 1996, by Compaq, Digital Equipment Corporation, IBM PC Company, Intel Corporation, Microsoft Corporation, NEC and Northern Telecom. When more than one USB device is coupled to a USB host controller, typically through hubs, the combined output of all the coupled USB devices cannot exceed 12 Mb/s. Thus, each individual USB device has a maximum transfer rate of less than 12 Mb/s.
When the devices connected to the USB host controller have very low-bandwidth requirements, the 12 Mb/s limitation total for all devices coupled to the USB host controller does not present a problem. However, when low-cost imaging devices which transfer significant amounts of data are coupled to the USB host controller, the imaging devices use a substantial portion of the 12 Mb/s bandwidth of the USB host controller. One example of such a low-cost imaging device is a Pro Share.RTM. camera from Intel Corporation of Santa Clara, Calif. Coupling two or more imaging such devices to the USB host controller can result in data loss due to insufficient bandwidth in isochronous systems and data transfer delays in non-isochronous systems due to the 12 Mb/s limitation.
Thus, an improved method of coupling multiple USB devices to a high-speed interconnect bus is needed.